Method of depositing silver in an electrolytic anode and composition therefor



United States Patent METHOD OF DEPOSI'IING SILVER IN AN ELEC- TROLYTIC.AN ODE ANB CGRIPQSlT-EON THERE- FOR Application September 14, 1953,Serial No. 380,096

1-4 Claims. .(Cl. 117-227) No Drawing.

This invention relates to a new and improved method for including inelectrolytic electrodes an impregnating oil and a metal, particularlysilver.

' In a United States patent application, Serial No. 380,164 concurrentlyfiled by Neal J. Johnson and Milton Janes, there is set forth and taughtan improvement in electrolytic electrodes consisting essentially ofincluding therein a certain percentage of metallic silver for thepurpose :of reducing anode wear and lowering cell voltage. This silvertreatment is in addition to the known impregnation of such electrodeswith an oil such as linseed oil.

The common method of accomplishing such silver inclusion, and the usuallaboratory one, is by impregnating the anode with an ;aqueous silvernitrate solution, followed by drying and subsequent heating of the anodeat about a temperature of 250 .C. to reduce the nitrate to metallicsilver. This is necessarily in addition to the subsequent separate :stepof impregnating the anode with an oil and then drying the :oil. Thesilver nitrate method has the further disadvantage of not achievinguniform distribution of "the silver through the area of the electrode,the :silver concentration near the surface of the electrode sometimesbeing as much as 5 to .7 times "that at the center.

The object of this invention is to provide a method whereby acarbonaceuos eletrolytic anode may be impregnated with oil and silver insuch a manner that the distribution of the silver throughout the anodeis nearly uniform. A further object is to improve the present methodwhich necessitates distinct and separate steps for impregnating an anodewith both silver and oil.

;In the practice of my invention, -a suitable impregnating oil, such aslinseed oil or other vegetable oil, is mixed with a diluent which ismiscible with the impregnating oil and which is also miscible withsolution of silver'nitrate in a mutual solvent. ,Such compounds asmethyl isobutyl ketone, methyl ethyl ketone, acetone, or chloroform aresuitable diluents. A typical mixture would' be 70 per cent oil and 30per cent methyl isobutyl ketone, but the actual proportion used dependson the percentage of oil impregnation of the anode desired. To thismixture is then added a solution of a suit-able silver salt, such assilver nitrate, in a mutual solvent, the quantity of solution addedbeing that sufilcien-t to give the desired percentage of silver,preferably 0.4 to 5 per cent silver by Weight, in the total impregnatingsolution which, using silver nitrate, may be in the range of 0.5 to 6.0per cent silver nitrate by weight. This, in turn, will depend .on thepercentage of sliver it is desired to deposit. Another silver saltsuitable for the invention is silver perchlorate. The silver salt usedmust, of course, he soluble in the mutual solvent employed.

The selection of the proper mutual solvent is essential to the practiceof the invention. Such a mutual solvent must be a good solvent for thesilver salt and also be miscible with the impregnating oils commonlyused. I

Patented May 29, 1956 have found dimethyl formamide to be an excellentmutual solvent for the invention. it is a powerful solvent for inorganicsalts, particularly chlorides and nitrates, with :a :solvent action onsilver nitrate about the same as that of water. It is miscible with mostorganic liquids and the impregnating oils. Other nitrogen-containingmutual solvents which are suitable for the invention includebenzonitrile and .dimethylacetamide. Hexylene glycol is also suitablewhere the percentage of silver desired in the total impregnatingsolution is not above one per cent by weight. I

Using a solution as described above, mutual deposition of silver andimpregnating oil is possible by simply immersing the anode to be treatedin the solution. The time of immersion would vary with the size of theelectrode to be treated. Simple immersion as a method of impregnating istime-consuming, however, and the preten ed method comprises using apressure-vacuum cycle to accomplish the impregnation.

Using this preferred method, a typical procedure is as follows. Theanode to be treated is placed in a suitable pressure-tight vessel, airis evacuated from the vessel for 15 minutes or more, and an impregnatingsolution according to the invention is introduced into the evacuatedvessel so as to cover the anode completely with an excess ofimpregnating solution being introduced to allow for penetration into thepores of the anode. After the vacuum has been released, the vessel isallowed to stand at atmospheric pressure for from 1 to 2 hours. Thisstanding time, which constitutes the soaking or filling time, can beshortened by applying air or inert gas pressure, which method can reducethe filling time to about 30 minutes.

After completion of the vacuum-pressure cycle just described, thenow-impregnated anode is heated at a temperature of from to C. in asuitable vessel. The usual time of heating is from '3 to 4 hours. Thisheating reduces the silver salt to silver and permits recovery of thesolvent, an important economy of the process. By the method of theinvention just described, the extra operation involved in impregnatingand heating the anode twice, once for the silver salt and once for theoil, is entirely eliminated.

Another advantage of my method is the more uniform distribution of thesilver through the anode as well as the use of a smaller silver nitratepercentage in the impregnant, for the same quantity of silver deposited.By the old method, due to the poor distribution of the silver involved,in order to have a high enough percentage of silver in the interior ofthe anode, there was deposited far more silver than was necessary in theouter areas of the anode, with a great waste of silver resulting. Theseadvantages, as well as others, canbc seen from the tables which follow.

Table I summarizes tests made on three anodes :two inches in diameter,each with a dilferent percentage of silver nitrate in .the impregnatingsolution.- For each anode, animpregnating solution was made up by mixing3.0 per cent'by volume of methyl isobutyl ketone with 770 percent of :aliquid raw vegetable oil of uniform quality and similar in properties tolinseed oil, and then adding suflicient quantities of a 20 per :centsolution of silver nitrate in dimethyl forma-mide to give the desiredpercentage of silver nitrate in the impregnation solution. Afterimpregnation, eachanode was heated at about 175 C. to remove the:solvent :and reduce the silver nitrate to metallic silver. Progressivecuts were then made into the anodes at quarter-inch diameter intervals,and the percentage of metallic silver at each depth was measured andrecorded.

Table I g a in Anode N o. Impreg- Test Zone regent nating g SolutionOuterrnost, Quarter-inch 0. 074 Second, Quarter-inch. 0. 074 1 1. 5Third, Quarter-inch. 0. 081 Fourth, Quarter-inc 0.079 Fifth,Quarter-inch H 0. 056 outermost, Quarter-inch H 0.124 Second,Quarter-inch 0.116 2 2 Third, Quarter-inch 0.120 Fourth. Quarter-inch0.121 Fifth, Quarter-inch .1 0. 108 Outermost. Quarter-inch 0. 384Second, Quarter-inch". 0.348 3 6 Third, Quarter-inch 0. 348 Fourth,Quarter-inch .1 0. 361 Fifth, Quarter-inch 0. 861

Table II Zone Percent Ag outermost, A6 inch 2. 35 Second, 916 inch 0. 47Third, inch 0.31 Fourth, A6 inch 0.31

I claim: 1. In the manufacture of a carbonaceous electrolytic anodecontaining as impregnants both a fatty drying oil and silver, theimprovement which comprises preparing an impregnating solutioncontaining a fatty drying oil, a silver salt, and an organic solvent forsaid silver salt which solvent is also miscible with said oil,impregnating said anode with said impregnating solution and heating saidanode after impregnation for a time sufficient for the purpose ofremoving said solvent, reducing said silver salt to metallic silver anddrying said fatty drying oil.

2. In the manufacture of carbonaceous electrolytic anodes containing asimpregnants both a fatty drying oil and silver, the improvement whichcomprises preparing an impregnating solution containing a fatty dryingoil, a silver salt, and an organic solvent for said silver salt whichsolvent is also miscible with said oil, the quantity of said silver saltbeing such as to make the percentage of silver in said impregnatingsolution between 0.4 and 5% by weight, impregnating said anode with saidsolution and heating said anode at a temperature of from 150 to 175 C.,for a time sufiicient to remove said solvent, reducing said silver saltto metallic silver and dry said fatty drying oil.

3. An improvement according to claim 1 wherein the impregnation of saidanode is accomplished by immersing said anode to be treated in saidimpregnating solution for the time required by the size of said anodeand then heating said anode after impregnation for a time sufficient forthe purpose of removing said solvent, reducing said silver salt tometallic silver and drying said fatty drying oil.

4. An improvement according to claim 1 wherein the impregnation of saidanode is accomplished by placing said anode in a pressure-tight vessel,evacuating the air from said vessel, maintaining the vacuum for at least'15 minutes, introducing said impregnating solution into said vessel,releasing said vacuum, allowing saidanode to remain in said vessel atatmospheric pressure for from one to two hours, removing said anode fromsaid vessel and heating said anode at a temperature of from to C. forfrom threeto four hours for the purpose of removing said solvent,reducing said silver salt to metallic silver and drying said fattydrying oil.

5. An improvement according to claim 1 wherein the impregnation of saidanode is accomplished by placing said anode in a pressure-tight vessel,evacuating the air from said vessel, maintaining the vacuum for at least15 minutes, introducing said impregnating solution into said vessel,releasing said vacuum, introducing a pressure of air or inert gas aboveatmospheric pressure into said vessel, keeping said anode under saidpressure for at least 30 minutes, releasing said pressure, removing saidanode from said vessel and heating said anode at a temperature of from150 to 175 C. for from three to four hours for the purpose of removingsaid solvent, reducing said silver salt to metallic silver and dryingsaid fatty drying oil.

6. An improvement according to claim 1 wherein said solvent is anitrogen-containing organic compound selected from the group consistingof dimethyl formamide, benzo nitrile, and dimethyl acetamide.

7. An improvement according to claim 6 wherein said solvent is dimethylformamide.

8. An improvement according to claim 6 wherein said solvent is benzonitrile.

9. An improvement according to claim 6 wherein said solvent is dimethylacetamide.

10. An improvement according to claim 3 wherein said solvent is anitrogen-containing compound selected from the group consisting ofdimethyl formamide, benzo nitrile, and dimethyl acetamide.

11. An improvement according to claim 4 wherein said solvent is anitrogen-containing compound selected from the group consisting ofdimethyl formamide, benzo nitrile, and dimethyl acetamide.

12. An improvement according to claim 5 wherein said solvent is anitrogen-containing compound selected from the group consisting ofdimethyl formamide, benzo nitrile, and dimethyl acetamide.

13. An impregnating solution for impregnating electrolytic anodescomprising a solution of a fatty drying oil, a diluent, and a sufficientquantity of a silver salt dissolved in a solvent, which solvent ismiscible with said fatty drying oil, so as to make the percentage ofsilver in the impregnating solution between 0.4 and 5 per cent.

14. An impregating solution for impregnating electrolytic anodescomprising a solution of a fatty drying oil, a diluent, and a quantityof silver nitrate in a solvent, which solvent is a nitrogen-containingorganic compound selected from the group consisting of dimethylformamide, benzo nitrile, and dimethyl acetamide, sufficient to make thepercentage of silver in the impregnating solution between 0.4 and 5 percent.

References Cited in the file of this patent UNITED STATES PATENTS301,192 White July 1, 1884 656,651 Markey Aug. 28, 1900 754,114 AtkinsMar. 8, 1904 1,717,140 Brandenberger June 11, 1929 1,834,812 Warren Dec.1, 1931 1,903,860 Gockel Apr. 18, 1933 1,909,800 Barton May 16, 19332,441,945 Frolich et a1. May 25, 1948 2,468,402 Kreidl et al Apr. 26,1949 2,511,472 Kmecik June 13, 1950 2,662,036 Levi Dec. 8, 19532,666,803 Kurlandsky Jan. 19, 1954 FOREIGN PATENTS 311,766 Great BritainAug. 6, 1930

1. IN THE MANUFACTURE OF A CARBONACEOUS ELECTROLYTIC ANODE CONTAINING ASIMPREGNANTS BOTH A FATTY DRYING OIL AND SILVER, THE IMPROVEMENT WHICHCOMPRISES PREPARING AN IMPREGNATING SOLUTION CONTAINING A FATTY DRYINGOIL, A SILVER SALT, AND AN ORGANIC SOLVENT FOR SAID SILVER SALT WHICHSOLVENT IS ALSO MISCIBLE WITH SAID OIL, IMPREGNATING SAID ANODE WITHSAID IMPREGNATING SOLUTION AND HEATING SAID ANODE AFTER IMPREGNATION FORA TIME SUFFICIENT FOR THE PURPOSE OF REMOVING SAID SOLVENT, REDUCINGSAID SILVER SALT TO METALLIC SILVER AND DRYING SAID FATTY DRYING OIL.